Composite high voltage,high frequency insulator construction



Dec. 16, 1969 K. N. MATHES ET AL 3,484,542

COMPOSITE HIGH VOLTAGE, HIGH FREQUENCY INSULATOR CONSTRUCTION Filed Aug. 30, 1968 KENNETH N. MAT/7'55 HENRY 7T PLANT,

THE/R ATTORNEY United States Patent U.S. Cl. 174-142 7 Claims ABSTRACT OF THE DISCLOSURE A composite insulator construction for high voltage, high frequency (about 0.1 to 50 mHz.) use is described. A metallic structure for supporting an antenna, for example, is afiixed to a resin-bonded fiber-reinforced laminate structure, whereby mechanical stresses applied to the metallic structure are transmitted to some support means. A feed-through conductor connected to the metallic structure passes through the laminate structure to provide an electrical path as between signal transmitting means and the antenna being supported. Protective track and erosion resistant elastomeric material is provided to cover portions of the metallic and laminate structures.

This application is directed to a composite insulator construction as may be used, for example, to support a radio antenna. The invention described herein was made in the course of, or under, a contract with the Department of the Navy.

BACKGROUND OF THE INVENTION Insulators for power transmission lines wherein a combination of materials have been employed to enable the insulator structure to resist both mechanical loading, applied in tension or compression, and electrical stresses. Thus, in Patent No. 915,052Sweetland et al., issued in Great Britain, a rod, or tube, of resin-bonded glass fiber is supplied with a closely-fitting longitudinally continuous covering of a relatively non-tracking, elastomeric, polymeric insulating material extending over the whole or major part of the length of the rod, or tube.

The term tracking is defined as the formation of dendritic (tree-like) tracks of carbon on the surface of a plastic insulator. This carbon formation results from the breakdown of the plastic material due to the impression of electrical discharges or scintillation across the insulator. The stress required to initiate tracking varies with the particular plastic material, the frequency of the applied electrical discharges, the weather and the degree of atmospheric pollution.

Electrical erosion refers to the removal of material from the insulator under the influence of electrical discharge or scintillation. This phenomenon may occur together with or separately from tracking.

Such structures have already been shown to be capable of sustained operation in applications in which high voltage power is being transmitted, and in which the power frequencies are typically 60 cycles per second. There does not appear to have been any information in the art of whether any of the materials displaying satisfactory track resistance at 60 c.p.s. would provide the necessary trackresistance and electrical erosion resistance when subjected to high frequency voltages, that is, for example, frequencies in excess of about 0.1 mHz. Thus, for example, shipboard whip antenna insulators, which may be simultaneously subjected to a variety of very severe mechanical, electrical and environmental conditions have heretofore been constructed of procelain. As is well known, porcelain shatters very easily upon impact. Also, procelain is weak in shear and relatively Weak in tension. In applications such as the aforementioned shipboard whip antenna in Which the insulator is usually mounted at the top of a mast and in turn supports an upwardly extending antenna, weight is a very important consideration and the heavy Weight of procelain insulators is a definite disadvantage.

There is, therefore, a recognized need for a light-weight, high-strength insulator able to transmit severe mechanical stresses over long periods of time, while transmitting very high frequency signals without electrical degradation.

SUMMARY OF THE INVENTION The above-noted need has been met with the structure disclosed herein, which has the additional feature of protecting against the transmission therethrough of such externally applied spurious electrical stresses as corona or arching caused by atmospheric electricity.

The structure of this invention employs in combination an inner supporting structure for transmitting both mechanical stresses and high frequency electrical signals, a central supporting structure affixed to the inner supporting structure, which transmits mechanical stresses received from the inner supporting structure to outwardly disposed support means. The central support structure iS constructed of a resin-bonded fiber-reinforced laminate and the outer surfaces of both the inner and central supporting structures are substantially covered with elastomeric material found to display track-resistance and electrical erosion resistance under exposure to high frequency electrical signals.

BRIEF DESCRIPTION OF THE DRAWING The exact nature of this invention as well as other objects and advantages thereof will be readily apparent from consideration of the following specification relating to the annexed drawing, which is a sectional view taken through the insulator construction of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The structure shown in the drawing contains features particularly advantageous for a light-weight insulator for a shipboard whip antenna. Typically, antenna 11 is affixed to and supported on insulator 12, which in turn is affixed to a mounting flange 13 at the top of a mast (not shown).

Antenna 11 is fastened to the inner supporting structure of the insulator 12 by means of bolts 14, the inner supporting structure being shown as a hollow metal welded cylinder consisting of conductor tube 16, bottom plate 17 and upper plate 18, all preferably of No. 316 Series stainless steel. Metal well cups 19 are affixed to upper plate 18 as by welding, one well cup being located under each threaded bolt hole 21. Well cups 19 serve to accumulate any moisture, which may find its way through bolt holes 21 thereby preventing the entry thereof to the interior of the hollow cyclinder. The feed-through conductor 22 shown threaded at its lower end is aflixed to base plate 17, preferably by welding, and provides the necessary conduction to complete the path for electrical signals comprising antenna 11, upper plate 18, conductor tube 16, bottom plate 17, feed-through conductor 22 and signal producing apparatus (not shown).

The central supporting structure of insulator 12 comprises fiber-reinforced resin laminate 23 with backup plate 24 through which fasteners 26 project to threadably engage bottom plate 17 of the inner supporting structure. The outer exposed annular portion, or flange, of laminate 23 serves as the flange of insulator 12 by which the insulator is mounted to plate 13 (as by bolts 27). Metal annular pieces, or rings 28 and 29 disposed to either side of laminate 23 are cemented to and reinforce the flange of insulator 12. The inner edges of rings 28, 29 are curved or radiused as at 28a, 29a preferably extending in an arc of 180 or more. As with most of the metallic components of insulator 12, fabrication from stainless steel is preferred although other common metals may be employed.

Materials for the construction of laminate 23 comprise low dielectric loss fiber reinforcement, as for example glass or fused silica fibers, and a low dielectric loss polymer, as for example electrical grade styrene-polyester or epoxy resins. Preferably, fiberglass cloth and polyester resin are employed in the preparation of laminate 23 taking care to minimize air entrainment.

The protective deposits of track-resistant insulation 31, 32 are preferably made of room temperature curing silicone rubber to which a filter, for example, hydrated alumina, aluminum oxide, magnesium hydrate or magnesium oxide has been added. However, filled ethylenepropylene elastomers may also be employed. The outside sloping surface of each of insulation coverings 31, 32 is smooth to minimize the collection of water and dirt. The dimensions of insulator 12 (height and diameter of conductor tube 16 and the diameter of laminate 23) as well as angles on and {3 are selected to provide some sufficiently long surface path extending between antenna 11 and ring 28 to provide requisite creepage distance for resistance to surface tracking. In an exemplary construction:

angle a=15 height of conductor tube 16:6 inches;

thickness of laminate 23=1% inches;

overall height of the insulator (from the top of conductor tube 16 to the apex of 32):about 12 inches.

All metal surfaces, which would be in contact with the track-resistant material should be prepared by grit blasting before casting bodies 31 and 32 in place.

The tendency for plastic insulating materials to fail electrically along surfaces exposed to moisture and contaminants continues to present a series problem in the dessign of high voltage outdoor electrical apparatus. Although considerable progress has been made in the development of track resistant plastics for high voltage outdoor applications at 68 Hz. (60 c.p.s.), which is the typical power frequency, such has not been the case for high frequency applications. When track-resistant resins are exposed to frequencies in the order of 2 mHz., it has been found that electrical erosin becomes a serious problem Thus, track-resistant materials, which display minimal erosion at 60 Hz., proved to be unsuitable as track-resistant materials at 2 mHz., because of ineffective resistance to electrical erosion. For example, polytetrafluoroethylene erodes least of all at 60 Hz., while being very badly eroded at the higher frequency. Also, the character of the erosion of these track-resisting materials is often different at the two frequencies. At 60 Hz. the erosion tends to occur in separated discrete areas, while at 2 mHz., a much more general erosion of the surface area occurs.

Arcing shield 33, which has a dish-like shape and is mounted with its flange directed toward insulator 12 for protection against continuous electrical discharges which are initiated by lightning, St. Elmos fire or other electrostatic effects. This electrostatic arcing shield actually serves to keep such electrical discharges from contacting the insulation (cover 31) to cause degradation thereof or to produce radio interference. Arcing shield 33 is made of metal, e.g. stainless steel or aluminum.

Initially, metal flange rings 28 and 29 were not provided with the radius portions 28a and 29a and the inner diameters thereof were not buried in the encapsulating material. Under test, as the voltage applied via conductor 22, plate 17, tube 16 and plate 18 at 2 mHz. was increased to between 4 and 6 kv. (peak) continuous arcs originating at bolts 27 played over the adjacent insulation surfaces eroding them appreciably in a matter of minutes. By introducing radius portions 28a and 29a and extending these radiused inner edges into encapsulating insulating bodies 31 and 32 it was found that no arcing occurred even up to voltage applications of 15 kv. (peak) at 2 mHz.

Thus, with the insulator construction disclosed very reliable performance may be obtained in the frequency range of from about 2 to about 32 mHz. under extremely severe conditions such as are encountered on shipboard. Such an insulator construction is about half as heavy as porcelain-type insulators used for the same purposes and in contrast to porecelain, has considerable resistance to shock.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. An electrical insulator construction for mounting an antenna for broadcasting and receiving radio frequency signals, said insulator construction simultaneously supporting said antenna under applied mechanical stresses and providing the necessary electrical contact between said antenna and the transmitting and receiving equipment served thereby, said insulator construction comprising in combination:

(a) a fiat fiber-reinforced laminate body,

(b) a hollow metallic enclosure comprising a pair of spaced parallel faces and connecting wall area extending therebetween,

(1) said enclosure being aifixed along one of said pair of faces to one of the two major surfaces of said laminate body, said laminate body extending laterally beyond said one of said faces,

(2) the second of said pair of faces being adapted for the mounting of an antenna thereto,

(c) a first deposit of track-resistant elastromeric material covering and bonded to the exterior of said connecting wall area and to a portion of said one major surface of said laminate body,

(d) a second deposit of track-resistant elastomeric material covering and bonded to a substantial portion of the other of said two major surfaces of said laminate body, and

(e) an electrical connector extending from said one of said faces through said laminate body and said second deposit.

2. The electrical insulator construction as recited in claim 1 wherein the laminate body is made of glass fiberreinforced polyester resin and the hollow metallic enclosure is a right cylinder.

3. The electrical insulator construction as recited in claim 2 wherein the deposits of track-resistant material each have a smooth outer surface and are made of filled silicone rubber.

4. An electrical insulator construction for mounting an antenna for broadcasting and receiving radio frequency signals, said insulator construction simultaneously supporting said antenna under applied mechanical stresses and providing the necessary electrical contact between said antenna and the transmitting and receiving equipment served thereby, said insulator construction comprising in combination:

(a) a flat fiber-reinforced laminate body,

(b) a hollow metallic enclosure comprising a pair of spaced paralled faces and connecting wall area extending therebetween,

(1) said enclosure being affixed along one of said pair of faces to one of the two major surfaces of said laminate body, said laminate body extending laterally beyond said one of said faces,

(2) the second of said pair of faces being adapted for the mounting of an antenna thereto,

(0) a first deposit of track-resistant elastomeric material covreing and bonded to the exterior of said connecting wall area and to a portion of said one major surface of said laminate body,

(d) a second deposit of track-resistant elastomeric material covering and bonded to a substantial portion of the other of said two major surfaces of said laminate body,

(e) an electrical connector extending from said one of said faces through said laminate body and said second deposit,

(f) a first metal flange covering the rim area of said one major surface of said laminate body, and

(g) a second metal flange covering the rim area of said other of said two major surfaces of said laminate body.

5. The electrical insulator construction as recited in claim 4 wherein the inner perimeter of the first and second metal flanges is radiused with the radiused portion extending into the first and second deposits of track-resistant material respectively.

6. The electrical insulator construction as recited in claim 5 wherein the laminated body is made of glass fiber- UNITED STATES PATENTS 1,565,799 12/1925 Dubilier 174-152 X 2,445,336 7/1948 Rauch.

FOREIGN PATENTS 342,416 10/ 1921 Germany. 531,435 1/ 1941 Great Britain.

LARAMIE E. ASKIN, Primary Examiner U.S. Cl. X.R. 

